Summary reports with Sub Summary layout parts are not supported by IWP. Upgrading to WebDirect is one option.
Another option that may serve is to set up a related table where you have one record for each sub summary group. You'll probably need to use a script to create those related records, but once you have them, aggregate functions like sum and count can compute the same totals for your groups of records being subtotaled with your sub summary layout part.
And even more elaborate option is to use a script that copies the data to a different table, adding a "sub head" record for each group where data fields are updated with the aggregate data that you want to show in your report.